Currently, ultra slimness of an organic light emitting diode (OLED) television (TV) and the like, aside from a portable terminal such as a mobile phone, a smart phone, a tablet personal computer (PC), and the like, has become a trend. Accordingly, a piezoelectric speaker capable of overcoming constraints in a thickness of an existing dynamic speaker using a magnetic coil is in the spotlight. Compared to the existing dynamic speaker, the piezoelectric speaker has advantages in being thin, light, and consuming low power. Accordingly, the piezoelectric speaker has been aggressively explored as a futuristic speaker capable of replacing the existing dynamic speaker.
The piezoelectric speaker does not use the magnetic coil, which is different from the existing dynamic speaker, and is driven based on a principle of aggressively employing resonance of a piezoelectric thin film itself. That is, due to contraction and expansion of a piezoelectric thin film occurring when an alternating current (AC) signal is input to the piezoelectric thin film, displacement occurs on a diaphragm and sound is reproduced by forming a dilatational wave in the air based on a mode-displacement characteristic according to a frequency. Based on the above principle, the piezoelectric speaker is driven. Accordingly, the piezoelectric speaker includes a plurality of resonant modes in a frequency response characteristic and thus, has a disadvantage in that a peak-dip or hump-hollow easily occurs. That is, due to the above peak-dip phenomenon of the frequency response characteristic, output of the piezoelectric speaker becomes unstable, distortion of sound becomes easy, and sound quality of sound reproduced is degraded.
As a conventional method for enhancing the above sound quality issue of the piezoelectric speaker, a method of bonding piezoelectric devices having different resonant frequencies on both surfaces of a diaphragm, respectively (Korean Patent Application No. 10-2010-0027915), a method of increasing a mass of a piezoelectric body by disposing a vibration adjusting portion at a center of a piezoelectric thin film (Korean Patent Application No. 10-2000-0032846 and U.S. patent application Ser. No. 12/023,496), and the like are proposed. However, the conventional methods adjust an amplitude by lowering a resonant frequency and thus, generally enhance a peak-dip in the lower register.